Yes, Steam. Not Windows 10. Steam.
Microsoft’s PC gaming initiative has begun to bear fruit a little, with the last few PC ports of their games actually being fairly good. There are, of course, still some problems- Microsoft’s Xbox games are only available on PC via the Windows Store, which is loathed by just about everyone who uses it, and they are tied to the UWP format.
Some Xbox games, such as Rise of the Tomb Raider, Quantum Break, and Dead Rising 4, have shown up on PC on Steam as Win32 exe files- but none that were actually developed by Microsoft’s internal studios. However, if this rumor on OnMSFT is correct, then we may be seeing some Xbox One exclusives make their way to PC- Xbox One exclusives that aren’t even available via the Windows Store yet.
According to the findings of SteamDB, a reputable site that keeps track of changes in the Steam database, listings for Halo 5: Guardians and Forza Motorsport 5 have been discovered within the Steam database. This seems to imply that the games might launch on Steam at some point in the future (possibly alongside, or after, a Windows Store release)- at the same time, however, it is important to remember that not all Steam database listings pan out. Remember that Halo 3 listing in the Steam database from all those years ago? Ten years later, Halo 3 is still an Xbox 360 exclusive.